Today, information networks are progressing, and the advancement of mobile electronic apparatuses such as cell phones, notebook PCs, digital cameras, and digital video cameras is remarkable. A battery is one of important parts for supporting high performance and maintaining high portability of these apparatuses. In particular, secondary batteries such as Nickel Cadmium (Ni—Cd), Nickel Metal Hydride (NiMH), and Lithium Ion (Li-ion) batteries are reusable by charging during the cycle life of each battery. Therefore, these secondary batteries are good for the environment because their waste amount is smaller than that of primary batteries which are discarded after being used once.
Secondary batteries are presumably rechargeable 200 to 500 times. In practice, however, a battery pack which is not repetitively recharged many times often becomes unable to be recharged, or, even when the battery pack is rechargeable, its voltage quickly decreases to shorten the time of usage before the next charging. In other words, a so-called drop in battery capacity is often experienced.
One cause is, for example, a phenomenon generally called a “memory effect” of the Ni—Cd battery and the like. If recharging is performed while the depth of discharge is shallow, a drop in voltage occurs early compared to the actual capacity of the battery, and the usable capacity decreases. However, this memory effect can be recovered by a refreshing operation which discharges the battery to a voltage requiring recharging.
Unfortunately, even though this refreshing operation is performed, a battery pack operates worse and the life of the battery is shorter than expected in some cases.
The present inventors, therefore, examined unrechargeable battery packs by disassembling them and found that some or all of a plurality of cells forming an unrechargeable battery pack have an extremely low terminal voltage or opposite polarities. That is, so-called pole reversal occurs to make it impossible to hold the charging voltage of the battery pack. Consequently, even when the battery pack is attached to a charger, a protection circuit of the charger operates to make charging impossible. This phenomenon is disclosed in Japanese Patent Publication No. 2732204 and Japanese Patent Publication No. 2743155 (paragraphs 8 to 23).
In principle, no pole reversal occurs if a battery pack is formed by secondary battery cells having exactly the same characteristics. In reality, however, it is impossible to make such secondary battery cells having exactly the same characteristics. Pole reversal unavoidably occurs in a battery pack which obtains a desirable voltage by connecting a plurality of batteries in series. As a method of solving this problem, a technique disclosed in Japanese Patent Laid-Open No. 7-327323 is proposed. This technique relates to a battery apparatus having a means for switching the connections of a rechargeable battery forming a battery pack. As shown in FIG. 1, a desired voltage is obtained during discharge by connecting cells in series, but these cells are connected in parallel during charging.
The above method of switching the connections has the following drawbacks:
(1) Even if dendrite grows in only one of the secondary battery cells forming a rechargeable battery to cause inconveniences (e.g., an internal electrode short circuit), other parallel-connected normal secondary battery cells discharge. In this case, no charging voltage can be applied to these parallel-connected normal cells, so the battery pack cannot be charged.
(2) Because the characteristics of cells forming a rechargeable battery are not exactly the same, variations in internal resistances of these cells make the effective voltages applied to the cells unequal to each other even when these cells are connected in parallel. That is, a cell having a higher internal resistance is fully charged later than the other cells. Accordingly, if charging is continued until this cell having the highest internal resistance is fully charged, cells having low internal resistances are overcharged. In contrast, if charging is terminated when a cell having a lower internal resistance is fully charged, cells having high internal resistances are used up as they are insufficiently charged. These insufficiently charged cells are connected in series during discharging and completely discharge first, allowing the easy occurrence of pole reversal.